BoronPlus planar diffusion sources have found widespread application in many semiconductor processing operations since they were first introduced to the industry in 1975. The increasing popularity of the sources can be attributed in part to the desirable combination of properties they exhibit including the unique diffusion-controlled evolution rate of B₂O₃ during use. The BoronPlus sources require a minimum amount of handling during use and produce a pure deposition on large diameter silicon wafers with excellent uniformity. In general, the BoronPlus sources exhibit a minimum of the undesirable characteristics of various other gas, liquid, and solid boron sources while retaining many of their desirable features.

Five Sources Meet Varying Needs

Five BoronPlus sources have been developed to meet the variety of needs in silicon processing. The useful operating temperatures for each source is given below. The overlapping temperature ranges provide the diffusion engineer flexibility in selecting diffusion sources for various applications.

Each source is produced from a glass containing B₂O₃ and the extremely stable oxides of BaO, MgO, Al₂O₃ and SiO₂. The glass composition is held within tight limits to assure melt-to-melt uniformity. The raw materials are melted and cast into billets utilizing a unique glass manufacturing process that ensures a homogeneous distribution of boron oxide throughout the bulk of the material. Each glass billet is subsequently nucleated and crystallized in a uniform way to provide the necessary high temperature rigidity to the sources. The billets are then turned to the desired diameter and sliced into wafers using a conventional ID saw.

BoronPlus Sources Have High Purity

All five BoronPlus diffusion sources are produced from high-purity raw materials. A typical impurity analysis of a melt, when measured on a spark source mass spectrograph, is given in Table I.

Figures 1 and 2 show relative amounts of sodium and iron found on the surfaces of silicon wafers doped with a BoronPlus source, a BN source, and a typical spin-on solution of B₂O₃ detected with a secondary ion mass spectrometer (SIMS). Although absolute amounts of impurities were not determined by this technique, the relative concentrations of impurities can be estimated by comparing the areas under the curves. These data indicate that most of the impurities are strongly tied up in the glassy matrix of the BoronPlus sources and do not evolve at a high rate during use. The result is a relatively pure glassy film of B₂O₃ being deposited on the silicon wafer surface.

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